(1) Field of the Invention
The present invention relates to an image reading apparatus using a monochrome image sensor.
(2) Background Art
Conventionally, there are known examples of image reading apparatuses such as an image reading apparatus that reads an image from an original copy using a monochrome image sensor and then generates monochrome image data. Also, there is an image reading apparatus that reads an image from an original copy using a color image sensor and then generates color image data. These image reading apparatuses are incorporated, for example, in copying machines, fax machines and scanners.
Some image reading apparatuses switch allowing a choice between either a low resolution mode or a high resolution mode for outputting image data in low or high resolution, respectively, and make one of the modes operate according to a command signal input from an operating portion of a user interface.
An example of such conventional apparatuses having a plurality of modes is an apparatus that obtains image data in high resolution from an image sensor and generates image data in low resolution by skipping pixel data constituting the image data in high resolution.
Another example is an image reading apparatus which comprises an image sensor including a sensor having a plurality of light receiving elements in a primary scanning direction, a shift register for outputting the respective light reception signals obtained from the light receiving elements arranged at even-numbered positions among the light receiving elements constituting the sensor, and another shift register for outputting the respective light reception signals obtained from the light receiving elements arranged at odd-numbered positions, which generates, in the high resolution mode, image data in high resolution by using output signals from both of the shift registers, and generates, in the low resolution mode, image data in low resolution (specifically, half of the resolution in the high resolution mode) by using output signals from one of the shift registers.
However, a conventional image reading apparatus of the former example which lowers the resolution by skipping image data involves a problem that the image reading speed of the image sensor is not improved even in the low resolution mode so that an advantage by lowering the resolution cannot be fully obtained.
A conventional image reading apparatus of the latter example provided with two shift registers also involves a problem that the needs of users who demand a wide variety of selectable resolutions cannot be fully satisfied since the selectable resolutions are only two, although the processing speed can be effectively improved in the low resolution mode.
The inventors of the present invention devised an image reading apparatus by providing a conventional image reading apparatus of the latter example with an additional sensor disposed at a predetermined distance away from the above-mentioned sensor in a secondary scanning direction and having a plurality of light receiving elements whose light receiving positions are arranged between the respective light receiving elements of the above-mentioned sensor, and an additional shift register for outputting the respective signals obtained from the light receiving elements constituting the additional sensor.
Such an image reading apparatus is advantageous since the resolution can be switched among three selectable resolutions by combining two sensors and three shift registers and the image processing speed may be improved depending on the resolution.
For example, when each sensor is set to be able to read with a resolution of 600 dpi in the primary scanning direction, the image reading apparatus as above can generate image data with a resolution of 1200 dpi by using all the signals obtained from the three shift registers, generate image data with a resolution of 600 dpi by using the signals obtained from the additional shift register and generate image data with a resolution of 300 dpi by using the signals obtained from the shift register for outputting the respective light reception signals of the light receiving elements arranged at even-numbered (or odd-numbered) positions.
In the image reading apparatus as above, however, circuits required for finishing conversion of analog light reception signals obtained from the shift registers into pixel data as digital signals are increased and limitation to the installation location of the circuits is imposed. As a result, the length of the wiring in the circuits is increased, which makes the image reading apparatus susceptible to the influence of external noises.
An object of the present invention, which has been made to solve these problems, is to provide an image reading apparatus that reads an image by using a monochrome image sensor having three shift registers, which requires a reduced manufacturing cost and is less susceptible to external noises.